D-meson production in p-Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV and in pp collisions at $\sqrt{s}=7$ TeV

TL;DR

This article reports precise measurements of prompt D-meson production in pp collisions at $\sqrt{s}=7$ TeV and in p-Pb collisions at $\sqrt{s_{NN}}=5.02$ TeV with the ALICE detector, extending $D$-meson cross sections down to $p_T=0$ using a non-vertexing method and a vertexing method. The study finds $R_{pPb}$ for prompt D mesons to be consistent with unity across the full $p_T$ range, suggesting minimal cold-nuclear-matter effects and providing a robust baseline for heavy-ion (Pb-Pb) studies. Results are compared to pQCD frameworks (FONLL, GM-VFNS, $k_T$-factorisation) and transport models, generally agreeing within uncertainties and enhancing constraints on charm production at low $x$ and on possible final-state interactions in small systems. The work thus tightens the link between perturbative charm production and the observed nuclear effects, with implications for interpreting D-meson yields in heavy-ion collisions and in cosmic-ray physics relevant for charm-induced backgrounds.

Abstract

The production cross sections of the prompt charmed mesons D$^0$, D$^+$, D$^{*+}$ and D$_s$ were measured at mid-rapidity in p-Pb collisions at a centre-of-mass energy per nucleon pair $\sqrt{s_{\rm NN}}=5.02$ TeV with the ALICE detector at the LHC. D mesons were reconstructed from their decays D$^0\rightarrow{\rm K}^-π^+$, D$^+\rightarrow{\rm K}^-π^+π^+$, D$^{*+}\rightarrow D^0π^+$, D$_s^+\rightarrowφπ^+\rightarrow{\rm K}^-{\rm K}^+π^+$, and their charge conjugates. The $p_{\rm T}$-differential production cross sections were measured at mid-rapidity in the interval $1<p_{\rm T}<24$ GeV/$c$ for D$^0$, D$^+$ and D$^{*+}$ mesons and in $2<p_{\rm T}<12$ GeV/$c$ for D$_s$ mesons, using an analysis method based on the selection of decay topologies displaced from the interaction vertex. The production cross sections of the D$^0$, D$^+$ and D$^{*+}$ mesons were also measured in three $p_{\rm T}$ intervals as a function of the rapidity $y_{\rm cms}$ in the centre-of-mass system in $-1.26<y_{\rm cms}<0.34$. In addition, the prompt D$^0$ cross section was measured in pp collisions at $\sqrt{s}=7$ TeV and p-Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV down to $p_{\rm T}=0$ using an analysis technique that is based on the estimation and subtraction of the combinatorial background, without reconstruction of the D$^0$ decay vertex. The nuclear modification factor $R_{\rm pPb}(p_{\rm T})$, defined as the ratio of the $p_{\rm T}$-differential D-meson cross section in p-Pb collisions and that in pp collisions scaled by the mass number of the Pb nucleus, was calculated for the four D-meson species and found to be compatible with unity within experimental uncertainties. The results are compared to theoretical calculations that include cold-nuclear-matter effects and to transport model calculations incorporating the interactions of charm quarks with an expanding deconfined medium.

Figures (17)

• Figure 1: Distributions of the invariant mass for ${\rm D^0}$ (top left), ${\rm D^+}$ (top right), ${\rm D_s^+}$ (bottom left) candidates and their charge conjugates and of the mass difference for ${\rm D^{*+}}$ (bottom right) candidates (and charge conjugates) in the rapidity interval $|y_{\rm lab}|<y_{\mathrm{fid}}(p_{\rm T})$ in p--Pb collisions. The dashed lines represent the fit to the background while the solid lines represent the total fit function. One $p_{\rm T}$ interval is shown for each species: $1<p_{\rm T}<2~\mathrm{GeV}/c$ for ${\rm D^0}$, $5<p_{\rm T}<6~\mathrm{GeV}/c$ for ${\rm D^+}$, $8<p_{\rm T}<12~\mathrm{GeV}/c$ for ${\rm D_s^+}$ and $16<p_{\rm T}<24~\mathrm{GeV}/c$ for ${\rm D^{*+}}$.
• Figure 2: Distributions of the invariant mass for ${\rm D^0}$ (left column), ${\rm D^+}$ (middle column) candidates and their charge conjugates and of the mass difference for ${\rm D^{*+}}$ (right column) candidates (and charge conjugates) in p--Pb collisions in the rapidity intervals $|y_{\rm lab}| <0.1$ (top row) and $-0.8 < y_{\mathrm{lab}} < -0.4$ ($-0.7 < y_{\mathrm{lab}} < -0.4$ for $p_{\rm T}<5~\mathrm{GeV}/c$) (bottom row). The dashed lines represent the fit to the background while the solid lines represent the total fit function. One $p_{\rm T}$ interval is shown for each species: $2<p_{\rm T}<5~\mathrm{GeV}/c$ for ${\rm D^0}$, $5<p_{\rm T}<8~\mathrm{GeV}/c$ for ${\rm D^+}$ and $8<p_{\rm T}<16~\mathrm{GeV}/c$ for ${\rm D^{*+}}$.
• Figure 3: Product of acceptance and efficiency for ${\rm D^0}$ mesons as a function of $p_{\rm T}$ (left) and as a function of $y_{\rm lab}$ (right). In the left panel, efficiencies are shown for prompt ${\rm D^0}$ with (solid line) and without (dashed line) PID selection applied, and feed-down ${\rm D^0}$ (dotted line). In the right panel, the ${\rm Acc}\times\epsilon$ values are shown for prompt ${\rm D^0}$ mesons for the three $p_{\rm T}$ intervals considered in the analysis as a function of rapidity.
• Figure 4: Left: Examples of fits to ${\rm D^0}$ (top), ${\rm D^+}$ (middle) and ${\rm D^{*+}}$ (bottom) impact-parameter distributions in the $p_{\rm T}$ intervals $3<p_{\rm T}<4~\mathrm{GeV}/c$, $5<p_{\rm T}<6~\mathrm{GeV}/c$ and $6<p_{\rm T}<8~\mathrm{GeV}/c$, respectively. The curves show the fit functions describing the prompt, feed-down and background contributions, as well as their sum, as described in the text. Right: fraction of prompt ${\rm D^0}$ (top), ${\rm D^+}$ (middle) and ${\rm D^{*+}}$ (bottom) raw yield as a function of $p_{\rm T}$ compared to the FONLL-based approach. The results from the data-driven method are shown as square markers with the error bars (boxes) representing the statistical (systematic) uncertainty. The arrow in the interval $1<p_{\rm T}<2~\mathrm{GeV}/c$ represents the minimum value within a 95% confidence level. The central values of $f_{\rm prompt}$ from the FONLL-based approach are shown by the dashed line and their uncertainty by the red boxes.
• Figure 5: Invariant-mass distributions of ${\rm D}^0 \to {\rm K}^-\pi^+$ candidates (and charge conjugates) in pp collisions at $\sqrt{s}=7~\mathrm{TeV}$ for two $p_{\rm T}$ intervals: $0<p_{\rm T}<1~\mathrm{GeV}/c$ (top panels) and $1<p_{\rm T}<2~\mathrm{GeV}/c$ (bottom panels). For both $p_{\rm T}$ intervals, the left panels display the invariant-mass distribution of all ULS K$\pi$ pairs together with the background distributions estimated with the LS, event-mixing and track-rotation techniques. The middle and right panels show the invariant-mass distributions after subtraction of the background from the track-rotation and LS techniques. Fit functions are superimposed.